The present invention relates to navigation satellite receivers, and more particularly to methods and systems for improving the time-to-first-fix of navigation satellite receivers by reducing the time uncertainty of just-started receivers.
Global positioning system (GPS) receivers use signals received from several earth-orbiting satellites in a constellation to determine user position and velocity, and other navigational data. A prior art navigation receiver that has just been turned on does not yet know where it is, how much its crystal oscillator is in error, nor what time it is. All these are needed to find and lock onto the satellite transmissions, and so a search must be made of the possibilities. The most probable are searched first to save time.
High-sensitivity GPS receivers are a problem when the initial time or frequency uncertainty is large. Finding signal energy when the signal energy is extremely faint requires making smaller steps and dwelling at each step longer. So having a better initial estimate of the local reference oscillator can improve time-to-first-fix.
GPS receivers with signal levels better that xe2x88x92145 dbm can readily lock onto a strong GPS satellite vehicle (SV) to decode the NAV-data. Such yields the SV ephemeris and position. After than, the total pseudorange needs to be formed from the hardware codephase. Conventional GPS receivers determine the integer millisecond and so-called z-count.
When signal levels are roughly no better than xe2x88x92145 dbm to xe2x88x92150 dbm, a practical high-sensitivity GPS receiver can employ pattern-matching tricks to get a z-count or integer millisecond for an anywhere-fix.
GPS receivers that are locked onto and tracking one or more GPS satellite vehicles (SV""s), know time to very high accuracy. This is because the GPS system is based on atomic clocks that set the time and frequency references used. The coarse acquisition (C/A) transmitted by the SV""s repeats every one millisecond of propagation wavelength, and so there is a basic ambiguity as to which millisecond a GPS receiver is observing. If the integer millisecond is known, e.g., time is known to better than one millisecond, then the integer ambiguity does not need to be solved. The z-count is known. Skipping the steps to find z-count and set the integer millisecond can save a tremendous amount of time and effort in a GPS receiver working on providing its first navigation solution fix after a cold start.
It is therefore an object of the present invention to provide a real time clock for faster initialization of a navigation satellite receiver by keeping track of time after the unit is powered-down.
It is another object of the present invention to provide a method and system for shortening the time needed for initialization of navigation devices.
It is a further object of the present invention to provide a satellite-navigation system that is inexpensive.
Briefly, a navigation-satellite receiver embodiment of the present invention comprises a real-time clock that can be slaved to the highly accurate time base of the GPS system. During such times, the amount of correction and the operating temperature are both tracked. It is assumed the operating temperature will has the greatest influence of frequency errors later when the real-time clock cannot be slaved to the GPS time base. When the receiver is powered-down, the real time clock is nevertheless kept alive. Its free-running frequency is corrected for temperature. The next time the receiver is powered up, time accurate to better than one millisecond in a day can be obtained instantly for use in other receiver initialization procedures.
An advantage of the present invention is that a system and method are provided that produce faster initialization times in navigation satellite receivers.
Another advantage of the present invention is that a system and method are provided for avoiding the use of more difficult procedures and calculations in GPS receivers to find the z-count and integer millisecond.
These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments which are illustrated in the various drawing figures.